Device for influencing the flow-through of plastic material which is heated such that it can flow through a housing

ABSTRACT

A device for controlling the flow of plastics material, heated until flowable, through a housing, in particular a filtering device or a direction-control device for the plastics material, comprises a flow channel extending through the housing, and a plunger of circular cross-section longitudinally displaced in a bore of the housing transversely to the longitudinal direction of the flow channel. The plunger crosses the flow channel. An unslotted scraper ring is mounted outside the housing on a portion of the plunger projecting from the housing and is held by a stop so that the plunger is displaceable relative to the scraper ring. The inner diameter (d 2 ) of the scraper ring is smaller than the diameter (d 0 ) of the bore of the housing and also smaller than the diameter (d 1 ) of the end of the plunger portion remote from the housing in the coldest operating state of that plunger portion. The scraper ring is always pretensioned on the plunger portion and therefore reliably scrapes off leaked material adhering to and carried by the plunger when the latter moves out of the housing.

BACKGROUND

1. Technical Field

The invention relates to a device for controlling the flow of plasticsmaterial, heated until flowable, through a housing, in particular afiltering device or a direction-control device for the plasticsmaterial, comprising a flow channel extending through the housing, and aplunger of circular cross-section longitudinally displaced in a bore ofthe housing transversely to the longitudinal direction of the flowchannel and crossing the flow channel, wherein an unslotted scraper ringis mounted outside the housing on a portion of the plunger projectingfrom the housing, a stop connected to the housing being provided for thescraper ring so that the plunger is displaceable relative thereto.

2. Related Art

Various types of these devices are known. One example is a filteringdevice for plasticised thermoplastic material, in which the plungercarries a filter arranged in the flow channel when the device is in theoperating position, the filter filtering out impurities from theplasticised plastics material. Another type is a device for changing thedirection of flow of the plastics material. In this case, the flow ofthe plastics material through the housing is always controlled by thelongitudinal displacement of the plunger. In the case of a filteringdevice, the aforementioned displacement of the plunger moves the filterinto a screen-changing position or a backwashing position so that thefilter can be replaced or cleaned (e.g. AT 395 825 B, EP 250 695 B). Inthe case of a direction-control device, the displacement of the plungercauses the plastics material introduced into the housing to leave thehousing through a different outlet opening than during the normaloperating state or, for example in the case of a gate valve, theplastics material is totally prevented from flowing through the housing.

In devices of the described type, plastics material unavoidably escapesfrom the housing during the longitudinal displacement of the plunger,even though the plunger is displaced in the housing with minimumclearance to ensure as little leakage as possible. The plastics materialforcibly drawn out of the interior of the housing during the movement ofthe plunger is decomposed by heat and the effect of atmospheric oxygen.The resulting carbonized mass forms a hard layer which surrounds theplunger and which has substantially lost its plastics character. Thepurpose of the scraper ring is to scrape this hard layer of leakedmaterial off the plunger during its longitudinal displacement relativeto the housing. However, tests have shown that the followingdifficulties arise here:

The portion of the plunger projecting from the housing is not heated andis therefore subject to a temperature decrease of approximately 100° C.over its axial, non-heated length. This means that the diameter of theplunger is smaller at its end projecting, from the housing than in theremaining portion by some tenths of a millimeter, depending on the sizeof this diameter, owing to the reduced thermal expansion in accordancewith its lower temperature. Therefore, when the plunger moves into theheated housing, the burnt-on layer of material is scraped off by thehousing edge or the scraper ring only until it corresponds to the borediameter of the housing or the inner diameter of the scraper ring.Consequently, a layer of this material remains at the colder end of theplunger and is of a thickness conforming to the smaller diameterattributable to the reduced thermal expansion of this end of theplunger. During the dwell time of the plunger in the position in whichit is retracted into the housing, e.g. the filtering position in thecase of a filtering device or the normal position of thedirection-control device or the like, this plunger portion heats up tothe housing temperature and therefore expands accordingly. The coatingof carbonized plastic on the plunger is densified as a result. Thisuncontrollably impedes the movement of the plunger back out again,possibly even jamming the plunger and with it the entire device.

The object of the invention is to overcome these difficulties andimprove a device of the initially described type so that its operationis more reliable.

SUMMARY

The invention achieves this object by the inner diameter of the scraperring being smaller than the diameter of the bore of the housing and alsosmaller than the diameter of the end of the plunger portion remote fromthe housing and projecting therefrom in the coldest operating state ofthat plunger portion so that the scraper ring is always pretensioned onthe latter. This close fit of the scraper ring on the plunger means thatthe scraper ring is always pretensioned on the plunger, irrespective ofthe temperature-induced expansions or contractions which the plungerundergoes. Naturally, this means that corresponding tensile stressesoccur in the scraper ring, but selecting the material for the scraperring, in particular its heat resistance values, so that the tensilestresses occurring are absorbed without causing the scraper ring tofracture does not give rise to any difficulties because the temperaturesoccurring are at least substantially known.

According to the invention, the inner diameter of the scraper ring ispreferably smaller than the diameter of the bore of the housing by 0.12%to 0.15%. Experiments have shown this range to be suitable for mostapplications.

The amount of pretension with which the scraper ring is mounted on theplunger portion is to be selected in accordance with the prevailingconditions. Generally, within the framework of the invention, thearrangement is such that the scraper ring is mounted on the plunger withan average contact pressure of 1000-2000 N/cm².

According to a further development of the invention, a clearance of 0 mmto 4 mm, measured in the axial direction of the plunger and bounded bythe stop, is provided between the scraper ring and the housing. The zerovalue, i.e. no clearance, applies to devices used for low-viscosityplastics, e.g. PET premonomers or waxes or the like. With theseplastics, there is also the danger of material leaking out when theplunger is extended and there is high mass pressure in the housing. Inthis case, the scraper ring without axial clearance acts like a gland.For high-viscosity plastics, however, the aforementioned amount ofclearance between the scraper ring and the housing is possible and, inthe region of this axial clearance, leaked material can issue freelywhen the plunger is extended.

The subject of the invention is shown schematically in the drawings bymeans of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be understood that the drawings are provided for the purposeof illustration only and are not intended to define the limits of theinvention. The foregoing and other objects and advantages of theembodiments described herein will become apparent with reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings in which:

FIG. 1 shows a device, formed as a filtering device for plasticsmaterial, in section along the plunger axis with axial clearance for thescraper ring;

FIG. 2 shows a variant of the scraper ring, in section according to FIG.1;

FIG. 3 schematically shows the plunger extended from the housing and thescraper ring removed therefrom, in section according to FIG. 1; and

FIG. 4 shows an embodiment without axial clearance for the scraper ring,in section according to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the filtering device shown in FIG. 1, a housing 1 has a flow channel2 for a flowable mass to be filtered, in particular thermoplasticmaterial to be cleaned of impurities for recycling purposes. Thematerial to be filtered enters flow channel 2 through an inlet opening 4in the direction of arrow 3. A filtrate flows out of housing 1 throughan outlet opening 6 in the direction of arrow 5. A bore 7, crossing flowchannel 2, extends perpendicularly to the axial direction thereof. Aplunger 8 of circular cross-section is longitudinally displaceable inbore 7, but guided with a close fit. A nest of screens 9 is incorporatedinto a plunger 8 and has an inlet chamber 10 for the material to befiltered, chamber 10 having a larger diameter than flow channel 2 andmerging via a stepped shoulder 11 into a frustum-shaped outlet chamber12 leading to the outlet side of flow channel 2. A filter 13 forseparating the impurities from the supplied plastics material isarranged between inlet chamber 10 and outlet chamber 12. Filter 13 hasan actual filtering layer 14 which can be formed by a metal plateprovided with fine through openings dimensioned so that the impuritiesto be separated out are held back. This filtering layer 14 isstrengthened by a supporting plate 15 supported against stepped shoulder11. Supporting plate 15 has channel-type openings 16, the diameter ofwhich are substantially greater than that of the through openings of thefiltering layer 14.

An electric resistance heater 17, by means of which the device and alsothe plastics material flowing in the device can be held at the desiredoperating temperature, is arranged on the outer casing of the housing 1.

In order to be able to clean or replace filter 13, plunger 8,constituting a screen carrier, is displaceable in the axial direction(double arrow 18) in its guide formed by bore 7. A drive (not shown) isprovided for this purpose. As a result of this axial displacement, aportion 19 (FIG. 3) of the axial length of plunger 8 projects fromhousing 1 at least intermittently and is therefore not heated. PlungerPortion 19 therefore has a lower temperature than the portion of plunger8 inside housing 1, or assumes this lower temperature over the course oftime. Furthermore, as a result of the aforementioned axial displacementof plunger 8, plastics melt, in particular of film thickness, is drawnout of the interior of housing 1 through the unavoidable non-tightpoints and remains at least in part on the casing of plunger portion 19projecting from housing 1. Because portion 19 of plunger 8 projectingfrom housing 1 is exposed to atmospheric oxygen, the plastics melt onits casing is decomposed and, within a short time, carbonized.Consequently, it loses its thermoplastic properties and, during thecourse of time, forms a hard layer surrounding this plunger portion 19.It has hitherto been assumed that this layer is sheared off by a housingedge 20 when plunger 8 moves back into housing 1. However, this is onlyin small part the case because portion 19 of plunger 8 projecting fromhousing 1 is subject to a considerable temperature decrease over itsnon-heated length, this temperature decrease being approximately 100° C.when treating thermoplastic material. This means that an end 21 (FIG. 3)of plunger portion 19 projecting from housing 1 has a diameter which,owing to the reduced thermal expansion, is smaller by some tenths of amillimeter than the diameter of plunger portion 8 still in housing 1.Consequently, a layer of carbonized plastics material, the thickness ofwhich corresponds at any point to the aforementioned difference inthermal expansion, remains on the colder end 21 of plunger 8 when itmoves back into housing 1. However, when plunger 8 moves back intohousing 1, the previously cooler plunger portion 19 of plunger 8 isheated to the temperature of housing 1 and therefore expandsaccordingly. The coating of carbonized plastics on the periphery ofplunger 8 is densified by this plunger expansion and impedes or preventsthe re-emergence of plunger 8.

To overcome these difficulties, an unslotted scraper ring 23 is fixed toa side wall 22 of housing 1 facing the free end 21 of plunger 8 so thatit cannot move relative to side wall 22 or can only move within limitsduring the longitudinal displacement of plunger 8. In this way, scraperring 23 provides a scraping function, by means of which the plasticslayer, which is carried by plunger 8 when it moves out of housing 1, isscraped off plunger 8. To ensure this for all temperature conditions ofplunger 8 or its plunger portion 19, scraper ring 23 is mounted onplunger 8 with a pretension which exceeds all temperature-inducedexpansions and contractions. It has been demonstrated that it issufficient to dimension the inner diameter d₂ (FIG. 3) so as to be 0.12%to 0.15% smaller than the diameter d₀ of bore 7 of housing 1, in whichplunger 8 is guided, this diameter d₀ consequently also being providedat housing edge 20 of housing 1. The inner diameter d₂ of scraper ring23 is therefore with certainty smaller than the diameter d₁ of thecoolest point on plunger portion 19 (front end 21) when plunger 8 isextended. In this connection:

d ₁ =d ₀−(d ₀ ·α·Δt)

where d₀ has the meaning given above, α is the expansion coefficient forplunger 8 and Δt is the temperature difference between front end 21 andthe portion of the plunger 8 remaining in the housing 1. As shown inFIG. 3, this smaller diameter d₁ results in a clearance “s” for frontend 21 in relation to housing edge 20, wherein$s = \frac{d_{0} - d_{1}}{2}$

The contact pressure between scraper ring 23 and plunger portion 19 ofplunger 8 to be scraped should advantageously lie between 1000 and 2000N/cm². Care must be taken that the tensile forces occurring in scraperring 23 do not exceed the allowable heat resistance value of thematerial used for scraper ring 23.

Scraper ring 23 can be immovably fixed to side wall 22 of housing 1,e.g. by means of a screw connection 24 (FIG. 4). Scraper ring 23 is thenmounted on housing 1 in the manner of a gland. This type of constructionis suitable for low-viscosity plastics, e.g. PET premonomers or waxes.However, for high-viscosity plastics it is more advantageous to providescraper ring 23 with clearance, enabling it to move in the axialdirection of plunger 8 relative to housing 1. This clearance 26 can beseen from the embodiments according to FIGS. 1 and 2. It is limited by astop 25 held by the screw connection 24. A clearance of up to 4 mm hasproved advantageous. This clearance 26 enables leaked material to passthrough the unavoidable non-tight points between housing 1 and plunger 8at housing edge 20 in the direction of arrow 27 when plunger 8 moves outof housing 1. The embodiment according to FIG. 2 permits betterdischarge of this leaked material by providing scraper ring 23 with achamber 28, in particular when the device is arranged so that thelongitudinal axis of plunger 8 extends vertically and the leakedmaterial can thus flow downwards.

As a result of the close fit of scraper ring 23 on plunger 8, asmentioned hereinabove, scraper ring 23 scrapes the leaked materialcarried by plunger 8 from the plunger surface when plunger 8 moves outof housing 1. An edge 29, adjacent to housing 1, of a central opening 30(FIG. 3) in scraper ring 23 is chiefly effective here. Any materialstill remaining on the periphery of the plunger is scraped off plunger 8by an edge 31—remote from housing 1—of central opening 30 in scraperring 23 when plunger 8 returns, i.e. is retracted into housing 1.

The device does not necessarily have to be constructed as a filteringdevice. Plunger 8 can also have the function of a closing member, forexample, wherein the axial displacement of plunger 8 has the effect ofclosing off flow channel 2. A variant of this is to provide channels inplunger 8 so that, when the plunger 8 is in the normal operatingposition, the flow of the treated material through flow channel 2 is notimpeded, whereas in a different operating position of plunger 8, inwhich the latter is displaced axially in relation to housing 1, thematerial enters housing through inlet opening 4, but leaves housing 1through an opening other than outlet opening 6, the material flow beingdiverted accordingly by the aforementioned channels in plunger 8.

What is claimed is:
 1. A device for controlling the flow of plasticsmaterial, heated until flowable, through a housing, comprising: a flowchannel extending through the housing, and a plunger of circularcross-section longitudinally displaced in a bore of the housingtransversely to the longitudinal direction of the flow channel andcrossing the flow channel, wherein an unslotted scraper ring is mountedoutside the housing on a portion of the plunger projecting from thehousing, a stop connected to the housing being provided for the scraperring so that the plunger is displaceable relative thereto, wherein theinner diameter (d₂) of the scraper ring is smaller than the diameter(d₀) of the bore of the housing and smaller than the diameter (d₁) of anend of the plunger portion remote from the housing and projectingtherefrom in the coldest operating state of that plunger portion so thatthe scraper ring is always pretensioned on the latter and scrapes off aplastic layer carried by the plunger when it moves out of the housing.2. The device of claim 1, wherein the inner diameter (d₂) of the scraperring is smaller than the diameter (d₀) of the bore of the housing by0.12% to 0.15%.
 3. The device of claim 1, wherein the scraper ring ismounted on the plunger with an average contact pressure of 1000 to 2000N/cm².
 4. The device of claim 1, wherein a clearance of 0 mm to 4 mm,measured in the axial direction of the plunger and bounded by the stop,is provided between the scraper ring and the housing.
 5. The device ofclaim 2, wherein the scraper ring is mounted on the plunger with anaverage contact pressure of 1000 to 2000 N/cm².
 6. The device of claim2, wherein a clearance of 0 mm to 4 mm, measured in the axial directionof the plunger and bounded by the stop, is provided between the scraperring and the housing.
 7. The device of claim 3, wherein a clearance of 0mm to 4 mm, measured in the axial direction of the plunger and boundedby the stop, is provided between the scraper ring and the housing. 8.The device of claim 1, wherein the device is a filtering device for theplastics material.
 9. The device of claim 1, wherein the device is adirection-control device for the plastics material.